Position determination systems, such as a machine vision measuring system, are used in many applications. For example, wheels of motor vehicles may be aligned using a computer-aided, three-dimensional machine vision alignment apparatus and a related alignment method. Examples of 3D alignment are described in U.S. Pat. No. 5,724,743, titled “Method and apparatus for determining the alignment of motor vehicle wheels,” and U.S. Pat. No. 5,535,522, titled “Method and apparatus for determining the alignment of motor vehicle wheels,” both of which are commonly assigned to the assignee of the present disclosure and incorporated herein for reference.
To determine the alignment status of the vehicle wheels, some aligners use directional sensors, such as cameras, to view alignment targets affixed to the wheels to determine the position of the alignment targets relative to the alignment cameras. These types of aligners require a calibration process to determine the relative positions between the alignment cameras in order to accurately determine the position between the wheels on one side of the vehicle and the wheels on the other side of the vehicle.
According to one calibration method, a large alignment target is positioned in the field of view of the alignment cameras, typically along the centerline of the alignment rack, and away from the alignment cameras. Since each alignment camera views the same alignment target, the alignment target's position relative to each alignment camera can be calculated and the positional relationship between the alignment cameras can be determined. This is called a relative alignment camera position (RCP) calibration. A RCP transfer function is used to convert one alignment camera's coordinate system into the other alignment camera's coordinate system so that an alignment target viewed by one alignment camera can be directly related to a alignment target viewed by the other alignment camera. One approach for performing an RCP is disclosed in U.S. Pat. No. 5,809,658, entitled “Method and Apparatus for Calibrating Alignment cameras Used in the Alignment of Motor Vehicle Wheels,” issued to Jackson et al. on Sep. 22, 1998, which is incorporated herein by reference.
While RCP calibration is accurate, it requires special fixtures and a trained operator to perform. Thus, there is a need for a simpler calibration process for calibrating a position determination system. Approaches for self-calibrations have been proposed in a co-pending patent application entitled “Self-Calibrating, Multi-Alignment camera Machine Vision Measuring System,” by Jackson et al., Ser. No. 09/576,442, filed on May 22, 2000, and a co-pending patent application entitled “Self-Calibrating 3D Machine Vision Measuring System Useful In Motor Vehicle Wheel Alignment,” by Jackson et al., Ser. No. 09/928,453, filed Aug. 14, 2002, both of which are commonly assigned to the assignee of the present application, and are incorporated herein by reference.
These approaches, however, do not resolve a problem encountered by position determination systems. After a position determination system, such as a machine vision measuring system, is installed and calibrated, the system usually can work with only certain sizes of objects under test. For example, a 3D aligner uses alignment cameras to view alignment targets affixed to the wheels. As the alignment cameras have limited fields of view, the system can determine alignment target positions only for vehicles with specific sizes. If a vehicle is wider or narrower than the specific sizes, the targets will fall outside the cameras' fields of view and the aligner cannot measure the positions of alignment targets without moving the aligner to a new position where that the alignment targets can be properly seen by the alignment cameras. The removal and reinstallation of the aligner is troublesome and consumes time. In addition, after the aligner is reinstalled, it takes time to aim the cameras towards the alignment targets.
Thus, there is a need for a position determination system that is adapted to different sizes of objects under test without the need to reinstall the system.
There is another need for an automatic system having sensing devices that can locate the alignment targets automatically without human intervention.
There is still another need for a user interface to indicate the positions of the sensing devices and whether the alignment targets are properly within the sensing field of the sending devices.